Generally, an ultrasound diagnostic system transmits an ultrasound signal from the surface of a human body toward a desired portion within a target object. This is so that an ultrasound image of soft tissues or blood flow can be obtained through non-invasive means by using information obtained through ultrasound echo signals.
Compared to other medical imaging systems (e.g., X-ray diagnostic system, X-ray CT scanner, MRI and nuclear medicine diagnostic system), the ultrasound diagnostic system is advantageous since it is small in size and fairly inexpensive. Further, the ultrasound diagnostic system is capable of providing real-time display and is highly safe without any dangerous side effects such as exposure to X-rays, etc. Thus, it is extensively utilized for diagnosing the heart, abdomen and urinary organs, as well as being widely applied in the fields of obstetrics, gynecology, etc.
In particular, a 3D ultrasound diagnostic system acquires 3D ultrasound data (volume data) of a target object by using a probe. The 3D ultrasound diagnostic system is then configured to display a 3D ultrasound image of the target object on a display by converting conical coordinates of the acquired data to Cartesian coordinates suitable for display (scan conversion). The 3D ultrasound image is displayed based on the volume data consisting of 3D scalar values.
More particularly, actual volume data are obtained by sampling values corresponding to discrete locations in a continuous 3D space of a target object. Unit volume data comprising the sampled volume data are referred to as voxel. The data existing in the continuous 3D space should be sampled in a constant interval in order to store the data by reducing the amount of the data.
In a volume ray casting, which is typically used for volume sampling, a ray is cast from each pixel of a viewing plane to a target object and samples a color value and opacity in a constant interval, as shown in FIG. 1. Thereafter, the sampled color value and opacity are formed, thereby determining the color and opacity of a specific voxel. A 3D image can be produced through repeating the above process. The opacity reflects an optical effect such as scattering and absorption of the ray.
The domain of a function used for the voxel sampling is different from that of the original 3D space. As such, it is required to quantize the sampled volume data for displaying the sampled volume data in the domain of the continuous 3D space. This quantization process is referred to as the reconstruction process.
In the volume ray casting, the number of rays to be cast into the volume data is determined according to the desirable image resolution. Further, the volume data are uniformly sampled in a ray propagation direction. By doing so, a desirable image quality can be obtained. Also, since a tri-linear interpolation filter is used as a reconstruction filter in the volume ray casting, the volume ray casting has a good performance for orthographic and perspective projections.
However, since the volume data are sampled even for a space, which is unnecessary to form the 3D ultrasound image, in a constant interval in the conventional volume ray casting, there is a problem in that it is difficult to enhance the rendering speed. For example, in case of ultrasound data of a fetus, the skin of the fetus to be observed is separated from the lining of the uterus in a constant distance, while amniotic fluid is filled between the uterus and the fetus. The unnecessary space such as the amniotic fluid is sampled at a constant interval in the conventional volume ray casting. Thus, there is a problem in that the rendering speed becomes decreased.
Also, since the volume data are sampled at a constant sampling interval in the conventional volume ray casting, there is a problem in that objects, which are smaller than the sampling interval, can be excluded during the sampling process.